This application claims priority from JP 2003-336137 filed on Sep. 26, 2003, which is hereby incorporated by reference in its entirety.
The present invention relates to methods for making semiconductor devices, in particular, semiconductor devices with metal gate electrodes.
MOS field-effect transistors with very thin gate dielectrics made from silicon dioxide may experience unacceptable gate leakage currents. Forming the gate dielectric from certain high dielectric constant (K) dielectric materials, instead of silicon dioxide, can reduce gate leakage. As used herein, high-k dielectric means having a dielectric constant higher than 10. When, however, a high-k dielectric film is initially formed, it may have a slightly imperfect molecular structure. To repair such a film, it may be necessary to anneal it at a relatively high temperature.
Because such a high-k dielectric layer may not be compatible with polysilicon, it may be desirable to use metal gate electrodes in devices that include high-k gate dielectrics. When making a CMOS device that includes metal gate electrodes, it may be necessary to make the NMOS and PMOS gate electrodes from different materials. A replacement gate process may be used to form gate electrodes from different metals. In that process, a first polysilicon layer, bracketed by a pair of spacers, is removed selectively to a second polysilicon layer to create a trench between the spacers. The trench is filled with a first metal. The second polysilicon layer is then removed, and replaced with a second metal that differs from the first metal.
The use of polysilicon layers that are ultimately replaced by the replacement metal gate raises a problem. When the source and drains are implanted using the polysilicon layers as a mask, and those implanted regions are subsequently annealed, a silicide forms over the polysilicon. Since it is intended to replace this polysilicon, the polysilicon must be etched away. But the silicide acts as a block, preventing removal of the polysilicon underlying the silicide.
Thus, there is a need for alternate ways to form replacement metal gate electrodes.
Features shown in these figures are not intended to be drawn to scale.